Metal parts, for example, those used in jet engines are generally required to meet very precise tolerances. Damage to metal parts during use or during machining where a part is overmachined can prevent the part from falling within the set tolerances and require that the part be repaired or replaced. To repair such damage, new metal must be brazed to the surface of the part.
The new metal needs to have a composition similar to the base metal. Thus, diffusion braze fillers are combined with powder base metal to provide a composition which brazes to the base metal at a temperature lower than the melting point or softening point of the base metal.
Typically, this was done by forming a slurry which includes the powdered base metal, powdered diffusion braze filler and a binder which could be, for example, a methacrylate binder, an alginate binder or the like. These systems provide acceptable results. However, well defined geometries needed for some repairs were very difficult to obtain.
Further, slurries such as these are difficult to use. The binder system must be initially mixed. Then the precise amount of base metal and diffusion filler must be combined. This has a very limited shelf life. It cannot be mass produced for sale and subsequent use. It must be prepared by the actual user which creates the potential problem of human error.
Also, the boron typically used in the braze alloy could localize or puddle on the surface of the part. This weakens the base metal and can destroy the part. Slurries are also difficult to conform, resulting in poor ability for large build-up repairs.
Also, with oxygen sensitive alloys such as those that include titanium, aluminum, hafnium, and chromium, heating above 800.degree. F. can cause oxide formation. These oxides are not normally reducible in brazing furnaces. Most braze furnaces are designed to either operate in a vacuum or in a hydrogen atmosphere. However, there is frequently a trace amount of oxygen remaining in the furnace that can react with these metals. To avoid this problem, such alloys containing these oxygen sensitive metals are nickel coated prior to base metal repair. This nickel precoating is undesirable simply because it requires an extra step or even two extra steps frequently requiring masking of portions that are not to be nickel coated.